This invention relates generally to improved unitized in-line and delta-type configured electron guns for color cathode ray picture tubes, and is specifically addressed to an improved aperture electrode structure for such guns that provides enhanced picture resolution.
Electron guns in color cathode ray picture tubes commonly generate three electron beams developed by cathodic thermionic emission. The beams are formed and shaped by a tandem succession of electrodes spaced along the central axis of the gun. The electrodes cause the beams to be focused on multiple phosphor groups located on the faceplate of a tube. A prime objective in the design of such guns is to provide a relatively small beam spot size for enhanced picture resolution.
A serious problem arises in the design and construction of electron guns, especially unitized, in-line guns enclosed in the widely used narrow-neck tubes--a problem resulting from the fact that the electron beams must travel in close proximity because of the space restrictions of the narrow confines of the neck of the cathode ray tube in which guns lie. For example, the three beams of an in-line unitized gun are commonly about one-quarter of an inch apart. As a result of this propinquity, electric field vectors contiguous to adjacent beam apertures tend to coalesce or otherwise interact to exert an undesired effect on the contours of the beams. This "cross talk" can result in deforming the beams so that the beam spots become elliptical at their respective points of landing on the picture imaging plane of the faceplate. This ellipticity can be compensated for in some measure, but at the cost of an undesired increase in spot size and consequent reduction in resolution.
The problem of interaction of the fields contiguous to apertures of adjacent beams is particularly acute in what is commonly termed "the aperture electrode." The aperture electrode is essentially a flat sheet suitably apertured to form an electrode. As shown in FIG. 1, an aperture electrode for a three-beam unitized, in-line gun, for example, may comprise a thin, substantially rectangular member 2 having apertures 4 therein for passage of beams 6. Aperture electrodes may have strengthening flanges 8 to prevent distortion of the planar surface. In a delta-configured gun, an aperture electrode may comprise a thin, suitably apertured disc, as shown by FIG. 2. It should be noted that the aperture electrodes shown by FIGS. 1 and 2 are displayed in simplest schematic form without appurtenances such as support tabs for attachment to supporting multiforms.
Aperture electrodes are attractive in that they cost less to manufacture than the commonly used "cylinder" electrode which, as the name connotes, comprises one or more beam-passing tubes, or cylinders. Also, aperture electrodes offer an electro-optical benefit in that an aperture in such an electrode produces a lens which appears to the beam passing therethrough to be of larger diameter than the lens produced by the beam-passing aperture of a cylinder electrode.
However, these benefits are largely offset by the fact that the electric field vectors of aperture electrodes contiguous to each aperure tend to coalesce with the electric field vectors of at least one adjacent aperture to produce a form of aberration known as astigmatism. Astigmatism is a focus defect in which electrons in different axial planes come to focus at different points on the picture imaging plane, resulting in distortion of the beam spot. More particularly in the context of this disclosure, astigmatism is defined as a focus defect in which electrons lying in a plane passing through the axes of coplanar adjacent apertures will, in general, come to focus at a different point than electrons lying the same distance from the axes, but lying in planes transverse to the first plane. In the case of aperture electrodes, distortion to astigmatism manifests itself as a pronounced spot ellipticity at the beam landing point on the picture imaging plane.
The use of aperture electrodes in the prior art has been restricted primarily to single-beam electron guns because of the aforedescribed astigmatism problem. In an attempt to alleviate aberration in aperture lenses, apertures have been formed with tubular projections extending therefrom. However, depending upon the length of the tubular projections, such structures tend to lose the electro-optical benefit of the true aperture electrode in that they become, in effect, cylinder electrodes.
The use of aperture electrodes in a three-beam unitized electron is disclosed in U.S. Pat. No. 3,772,554--Hughes. The electrodes comprise the control and screen grid electrodes located in the prefocus section of a gun having a bi-potential main focus lens. Beam aberration normally attributable to aperture electrodes does not appear to be a problem because the aperture electrodes are located in the "low-level" zone of the gun where beam aberrating influences are minimal. Also, the very small size of the apertures results in beams of such small diameter in the prefocus section that there appears to be no appreciable coalescing of electric field vectors.
Attempts to use aperture electrodes in other zones of the electron gun, such as the main focus lens section, have resulted in the introduction of unacceptable beam distortion--a problem to which this present invention is addressed.
In U.S. Pat. No. 4,086,513, Evans discloses means which are said to at least partially compensate for the horizontal elongation of electron beam spots due to deflection defocusing effects of certain seld-converging yokes. According to Evans, this problem is present in television picture tubes having wide deflection angles; e.g., 90.degree.-110 of deflection. One embodiment according to Evans provides horizontal parallel plates positioned on opposite sides of each beam and extending towards the screen from one of the focusing electrodes. The plates provide for a pre-distortion, or a pre-shaping, of the beams which is alleged to be effective to provide sufficient astigmatism in the electron gun so that a focus voltage can be obtained that provides optimum focusing of the electron beam in both the vertical and horizontal directions. Another embodiment of the invention provides for strengthening the focusing field about a horizontal axis by the placement of vertically oriented plates on opposite sides of each aperture of an accelerating and focusing eletrode.